1. Field of the Invention
The present invention relates to an image reading apparatus.
2. Description of the Related Art
Image reading apparatuses for reading images on to-be-read-media, such as originals, receive light reflected from the to-be-read-media and convert the reflected light into an electrical signal by using light receiving elements in order to electrically process the images on the to-be-read media. Some conventional image reading apparatuses read images by using what is called a line sensor, which is a relatively low-cost image reading unit having light receiving elements arranged in a one-dimensional direction, or include a mechanism for conveying a to-be-read-medium at the time of image reading in order to improve read operability. Some of the image reading apparatuses having the mechanism for conveying a to-be-read-medium convey a to-be-read-medium while applying a pressing force to the to-be-read-medium in order to convey the to-be-read-medium more appropriately.
For example, a plate-like-medium conveying apparatus disclosed in Japanese Patent Application laid-open No. 2000-255827 includes a rotatable roller arm that supports a driven roller, and a torsion spring that is mounted on a support shaft of the roller arm. In this structure, a force in the rotational direction about the support shaft is generated on the roller arm due to the elastic force of the torsion spring, and the driven roller applies a pressing force to a to-be-read-medium according to the force in the rotational direction.
An image reading apparatus disclosed in Japanese Patent No. 4311660 includes roller pairs arranged on the upstream and downstream sides of an image reading unit. In each roller pair, a driven roller is biased toward a driving roller by a compression spring. The image reading unit is biased by a compression spring in a direction that the width of a gap between the image reading unit and images to be read is narrowed. The image reading unit includes bearing units for bearing roller shafts of the driven rollers. Therefore, when a sheet medium carrying an image to be read enters between the driven roller and the driving roller and then the driven roller is separated from the driving roller, the force is transmitted to the bearing unit and the image reading unit moves in the same direction.
An image reading apparatus disclosed in Japanese Patent Application Laid-open No. H10-190938 is structured such that a read driven roller is press fitted to a read driving roller by a spring. At the time of reading an original, the original is caused to pass between the read driven roller and the read driving roller and a pressing force is applied to the original by the read driven roller. Therefore, the original comes into close contact with a transparent glass of a back-side reading unit and a contact sensor reads image information on the original.
An image reading apparatus disclosed in Japanese Patent Application Laid-open No. 2004-297760 includes a pressing-force generation mechanism for pressing an original against an image sensor. The pressing-force generation mechanism includes a pressing member that rotates about a rotation axis while pressing an original against a glass surface of the image sensor. Therefore, when the image sensor reads image information on an original, the original is conveyed with a pressing force applied by the pressing member.
In the image reading apparatuses that read images with conveyance of to-be-read media, a roller applies a pressing force to the to-be-read-media during the conveyance as above in order to perform appropriate conveyance. In the case of use of a contact-type image reading unit that reads images by contact with the to-be-read-media, the image reading unit also applies a pressing force to the to-be-read-media during reading. In the image reading apparatuses that cause the image reading unit to apply a pressing force to to-be-read-media and read images while the image reading unit is in close contact with the to-be-read-media, because both of the image reading unit and the roller apply the pressing force to the to-be-read-media, the roller is generally mounted on the image reading unit.
In recent years, there is a demand for downsizing of image reading apparatuses in order to improve installability, operability, appearance, and the like. To meet this demand, each unit for conveying to-be-read-media is downsized. Therefore, the diameter of the roller is downsized similarly to the other units. However, if the diameter of the roller mounted on the image reading unit is downsized, the amount of protrusion of the roller with respect to the image reading unit is reduced, reducing stroke for applying a pressing force to to-be-read-media. In this case, the pressing force applied to the to-be-read-media is reduced and conveyance of the to-be-read-media may become unstable.